1. Field of the Invention
The present invention relates to percussion tools.
2. Prior Art
FIG. 9 shows an arrangement of a conventional percussion tool. The disclosed percussion tool comprises a spindle 1 extending in an axial direction, and a cylindrical hammer 3 coaxial with the spindle 1. A spindle cam 2, shaped into a groove, is formed on an outer surface of the spindle 1, while a hammer cam 4, shaped into a recess, is formed on an inside surface of the hammer 3. A steel ball 5, interposed between the spindle 1 and the hammer 3, engages with both the spindle cam 2 and the hammer cam 4. With this engaging mechanism, the hammer 3 can advance forward or retract backward with respect to the spindle 1 in the axial direction. A spring 6 is disposed behind the hammer 3 to generate a force pushing the hammer 3 in the axial direction toward the anvil 12. The anvil 12 is provided adjacent to and in front of the spindle 1 on an extension line of the axis of the spindle 1. A flange of the anvil 12, formed at the rear end thereof, is engageable with a bifurcated front end of the hammer 3. When an excessive load is applied to the anvil 12, the flange of the anvil 12 pushes the hammer 3 rearward in the axial direction against the elastic force of the spring 6. At the moment the engagement is released between the anvil 12 and the hammer 3, the hammer 3 causes a free (no-load) angular displacement with respect to the anvil 12 accelerating its angular speed until it hits the flange of the anvil 12 again, thereby realizing a well-known percussion operation.
As illustrated in FIG. 10, after an engaging edge of the hammer 3 is disengaged from one flange of the anvil 12 (shown in the left of the drawing), the hammer 3 causes a free rotation in an angular direction with respect to the anvil 12 until it hits another flange of the anvil 12 (shown in the right of the drawing). In this case, the hammer 3 advances forward along the lead of the spindle cam 2 and the hammer cam 4. Thus, the hammer 3 hits the anvil 12 with an oblique force F, which is divided into an angular component F1 acting as percussion force and an axial component F2 acting in the axial direction and normal to the angular component F1.
Furthermore, when the hammer 3 reaches a bottom dead center of the spindle cam 2 before it hits the anvil 12 as illustrated in FIG. 11, an axial component F2' is transmitted via the steel ball 5 to the spindle 1. Then, the force is transmitted from the spindle 1 to the anvil 12 via an abutting surface 15 where the spindle 1 and the anvil 12 are brought into contact with each other.
The angular component F1 or F1' is necessary to generate a force for rotating or fastening a screw 17 via a tip tool 16 into an opponent member 18. Meanwhile, the axial component F2 or F2' which does not contribute to the screw fastening operation is transmitted to the opponent member 18 by way of the anvil 12, the tip tool 16 and the screw 17, causing vibration of the opponent member 18. Thus, the axial component F2 or F2' becomes the main cause of percussion noises generated from the opponent member 18. The level of such percussion noises generated from the opponent member 18 possibly increases up to 75% of the total noise energy generated in the screw fastening operation. It is needless to say that such noises will deteriorate work efficiency and, therefore, should be decreased from the view point of prevention of public nuisance.
A technology for reducing noises in the percussion tools is, for example, disclosed in the Japanese Utility Model No. SHO 56-6293/1981. According to this technology, a plurality of resilient members, such as synthetic rubber, are interposed between percussion operating members and a casing in order to prevent vibration of the percussion operating members from transmitting to the casing. Furthermore, a hermetical chamber is defined around percussion hammers housed in the percussion tool. This hermetical chamber is filled with oil, thereby suppressing or absorbing hammer noises with the cooperative effect of the resilient member and the oil. However, this noise reduction mechanism is not effective for preventing the opponent member from generating noises, since the axial component of a percussion force transmitted from the hammer to the anvil is directly transmitted to the opponent member, without being effectively reduced or eliminated. The Unexamined Japanese Patent Application No. SHO 55-44136/1980 discloses an arrangement for providing a spiral spring connecting an output shaft of the percussion tool and a tip tool, for absorbing vibration occurring in an axial direction. However, this arrangement is disadvantageous in that the peak or maximum value of the fastening torque is lowered, resulting in remarkable deterioration of performance. Furthermore, the Unexamined Japanese Utility Model Application No. SHO 48-80199/1973 discloses an idea of interposing an elastic member between a spindle and an anvil. This is, however, not effective to prevent the axial force of the anvil from being directly transmitted to the opponent member; therefore, it is not possible to prevent the opponent member from causing noises in response to the percussion operation.